A lithium ion secondary battery has characteristic high energy density and electromotive power as compared to a lead storage battery or a nickel-hydrogen battery; therefore, a lithium ion battery has been widely used as a power supply for various portable devices which are demanded to be smaller in size and lighter in weight, and laptop computers. Such a lithium ion secondary battery is generally produced by a method in which a positive electrode plate formed of a positive electrode current collector coated with a positive electrode active material and a negative electrode plate formed of a negative electrode current collector coated with a negative electrode active material are laminated through a separator provided therebetween, and the resultant laminate of the positive electrode plate, the separator and the negative electrode plate is sealed within a casing together with an electrolytic liquid. Further, a lithium ion secondary battery is generally constructed such that electrode terminals connected respectively to the positive and negative electrode plates of the laminate protrude from the casing (see, for example, Patent Document 1).
The lithium ion secondary battery described in Patent Document 1 has a structure wherein an electrode assembly having a positive electrode with a positive electrode lead, a negative electrode and a separator is accommodated in a laminate casing, and an insulating cover is provided at one or both of a position between the positive electrode lead and the laminate casing and a position between the negative electrode lead and the laminate casing. Further, in the structure of the lithium ion secondary battery of Patent Document 1, the insulating cover is formed by bending an insulating material in a sheet form, whereby the positive electrode lead and the negative electrode lead are also bent accordingly. This structure improves the tightness of seal of the battery while suppressing the damages such as cracks caused to the laminate casing during the sealing under reduced pressure.